The proposed research is part of a program in comparative immunohematology that is investigating the immune responsiveness of chickens. Two aspects are currently receiving attention (1) the analysis of the sources, nature and fate of the antigen-responsive cells that are released into the blood stream following immunization with heterologous erythrocytes, and (2) the investigation of factors that enhance or suppress their formation in the immature chicken and embryo. The adoptive cell transfer technic is essential for the detection and assay of the blood-borne progenitors of hemagglutinin-forming cells, and for the in vivo cell interaction studies. A colony of inbred White Leghorn chickens is being developed for high degree of intrastrain histocompatibility which will be suitable for cell transfer experiments.